In a video display apparatus, scanning circuits are synchronized to a synchronizing component or sync derived from the input video signal. Hence, a video display apparatus which is operable at multiple horizontal scanning frequencies must be capable of synchronizing to a standard definition NTSC signal horizontal scanning frequency of nominally 15.734 kHz or to a high definition, Advanced Television Standards Committee, ATSC, signal having horizontal scanning frequency of nominally 33,670 kHz with 1080 active lines and interlaced scanning (1080I ). In addition to synchronizing to broadcast video signals, the apparatus may be required to display computer generated non-broadcast video signals, such as, for example, a super video graphics adapter signal or SVGA, having a horizontal frequency of 37,880 kHz.
Horizontal frequency oscillators employing phase locked loop control are widely known and used in video display apparatus. Dual and triple phase locked loops are also known and used to provide functional separation between potentially conflicting requirements of synchronization and scanning waveform generation. In a dual loop configuration, a first loop may be a conventional phase locked loop in which a voltage controlled oscillator output, or an output divided therefrom is compared with horizontal synchronizing pulses derived from the video signal to be displayed. The second phase locked loop, which for example, operates at the same frequency, compares the oscillator output from the first loop with a horizontal rate pulse, for example, a retrace pulse voltage derived from or representative of defection current flow. The error voltage from the second phase comparison is used to generate a width modulated pulse signal which determines the initiation of the deflection output device turn off, and subsequently, retrace initiation, or the phase of each line within the period of a vertical scan.
The response of the first phase locked loop may be optimized for fringe area reception of broadcast video signals suffering poor signal to noise ratios. Such signals suggest that the response of the first phase locked loop is relatively slow. Accordingly, the first loop may have a narrow bandwidth to optimize phase jitter reduction. However, since a video display apparatus is required to be operable with signals from a variety sources and with differing horizontal frequencies. The response of the first phase locked loop represents a compromise between a narrow bandwidth for minimized phase jitter and a wide bandwidth, fast loop response capable of rapid phase recovery. For example, a narrow bandwidth loop is suited to synchronization by low noise, non-broadcast computer generated signals, whereas and wide bandwidth, fast loop response, capable of rapid phase recovery is required for synchronization of video cassette recorder (VCR) replay signals where abrupt changes in horizontal sync. pulse phase, by as much as 10 microseconds may occurring between the beginning and end of the vertical banking interval. Hence tradeoffs in respective loop responses may be made to provide adequate weak signal performance without significant overall degradation of receiver performance. The second phase locked loop generally has a faster loop response. Accordingly, the second phase locked loop may have a wider bandwidth allowing it to track variations in the defection current due to horizontal output transistor storage time variations, or high voltage transformer tuning effects. Such tight tracking yields a straight, non-bending raster largely independent of beam current loading.
The use of voltage controlled oscillators for horizontal frequency signal generation is well known. It is known to employ an oscillator operating at a multiple of the input horizontal sync. frequency and to achieve synchronization by means of a down counter with a selectable divide by two stage. However, when input signals have non-integer horizontal scanning frequencies, simple halving or doubling of an oscillator count down ratio cannot readily provide synchronization. In addition, input signals that are subject to widely differing distortions necessitate differing processing characteristics to provide optimized display performance.